Field of the Invention and Related Art Statement
The present invention relates to a control apparatus for a position control motor, such as a brushless motor or a stepping motor, and an apparatus which implements position control while preventing the hunting of a stepping motor.
Conventional control apparatuses of this type for a position control motor like a brushless motor or a stepping motor include a typical open loop control apparatus for a stepping motor and a closed loop control apparatus for an AC servo motor, which performs position control using a position detector like an encoder or resolver, etc.
The control apparatus for a stepping motor can implement high-precision position control easily and at a low cost simply by switching one excitation sequence for the stepping motor to another, and has therefore been used widely.
That is, such an open loop control apparatus changes over the excitation of the motor in response to a position command regardless of the rotor position.
Because of the open loop control employed, however, once the stepping motor goes asynchronous due to the vibration of the stepping motor itself, variations of the load, external influence or the like, the stepping motor cannot follow positions specified by control commands, thus so-called stepping out occurs and the actual stop position is shifted from the position specified by the control command. Where high reliability is required, therefore, it is necessary to use a large motor which has a sufficient margin for the needed torque or to use another motor like a servo motor.
Further, the closed loop control apparatus detects the rotor position by means of the aforementioned position detector and feeds back this information. Normally, the phase of the excitation current of the motor is controlled so that the phase is in an excitation stable point which is ahead of the rotor position by an electrical angle of 90 degrees.
FIG. 11 illustrates the relationship between the rotor displacement and the generated torque for the open loop control apparatus when a constant electric current is supplied to the stepping motor. This relationship between the rotor displacement and the generated torque shows that the generated torque becomes maximum when the displacement reaches 90 degrees, that a torque which causes the rotor to get back to the original position is generated within .+-.180 degrees, and that in a range of over .+-.180 degrees, a torque which moves the rotor to a different stable point, not to the original position, is generated positional deviations occur.
When the rotor of the motor cannot respond to the change-over of an electric current, therefore, step out of the motor occurs, resulting in lower position control reliability.
As mentioned above, the closed loop control apparatus controls the phase of the excitation current of the motor in such a manner that the phase comes to an excitation stable point which is ahead of the rotor position by an electrical angle of 90 degrees. Since the rotor position does not coincide with the excitation stable point, the rotor cannot stop fully and generates small vibrations. As the level of the exciting current is controlled by feedback of the position, speed and so forth, this feedback control causes an inevitable delay which requires, for example, the loop gain to be adjusted.
In this case, the following shortcomings would arise.
(1) The closed loop gain should be adjusted with respect to a variation in the load. Stable control against such load variations is complicated.
(2) Microvibration (hunting) occurs when the rotor stops.
(3) Due to the inevitable feedback-originated delay in control, synchronism to a command may be lost.
For the conventional open loop control apparatus, the use of a large-capacity motor results in an increased motor size, which stands in the way of making the control apparatus itself compact. Even if the motor size is increased, step out cannot be eliminated completely so that the reliability is not still assured.
The closed loop control apparatus for a servo motor or the like, on the other hand, can eliminate step out of the motor. Because the level of the electric current which is supplied to a servo motor is generally controlled based on a deviation of the actual position from a commanded position, however, the control is complex, leading to an increased cost.
This closed loop control apparatus also suffers the occurrence of microvibration when the motor stops, and difficulty in keeping the stability of the control system with respect to load changes.